Many manufacturing processes benefit from in-process measurement of product composition or quality. Optical spectroscopy is one means to perform these measurements. For non-turbid liquids, transmission spectroscopy is a commonly used method. For many other materials, non-contact forms of optical spectroscopy (e.g. reflectance, fluoresce, Raman) are often suited to these applications.
Unlike laboratory measurements that are performed under controlled conditions, in-process or at-line measurements typically must contend with the existing conditions in the manufacturing environment. In general, the accuracy of compositional or quality information derived from a spectroscopic measurement is related to the accuracy with which the optical spectrum was measured. This in turn is influenced, in large part, by the design of the measurement head that illuminates the sample, collects the illumination reflected back or emitted by the sample under inspection, and then delivers that collected illumination to the instrument performing the optical spectrum measurement.
Many parameters relating to a measurement head influence the accuracy. For example, such parameters include the frequency of the reference measurement, the instrument and illumination source status, the sample geometry, the sample surface texture, the ambient illumination, the secondary illumination and any stray (or scattered) illumination. Each of these parameters is described more completely below.
Frequency of the reference measurement: The optical spectrum of a sample is typically computed as a ratio of the spectroscopic instrument's response to the sample divided by the instrument's response to a reference sample. Since both the instrument's response function and the illumination or excitation source change or drift over time, the longer the time interval between measuring the reference and measuring the sample, the larger the error in the measured optical spectrum.
Instrument and illumination source status: The ability to monitor the condition of the spectroscopic instrument, illumination source and other aspects of the measurement system is critical to the long-term function of the system in this application.
Sample geometry: The physical location and orientation of the sample relative to the measurement head influence the observed optical spectrum since the characteristics of the measurement head's illumination (intensity and spectral distribution) vary positionally. Thus, the degree to which the illumination characteristics vary spatially and the variability of the sample's position relative to the measurement head combine to influence the accuracy of the measured optical spectrum.
Sample surface texture: The sample's surface texture and the illumination-to-collection angle determine the amount of shadowing ‘seen’ by the measurement head. Thus, larger illumination-to-collection angle combined with variation in sample texture results in greater variation of the observed optical spectrum.
Ambient illumination: In order to acquire an accurate optical spectrum, the ratio of the illumination characteristics (intensity and spectral distribution) when measuring the sample to that when measuring the reference must be known. Any time-varying illumination from sources other than the measurement head will lead to errors in the measured optical spectrum.
Secondary illumination: Illumination that strikes the sample, reflects back onto a secondary surface (e.g. some part of the measurement head or any other surface in the vicinity of the measured sample), and then re-illuminated the sample results in errors in the measured optical spectrum.
Stray (or scattered) illumination: Illumination that scatters off of any surface (other than the sample) that is viewed by the illumination collecting optics (e.g. an optical window. This results in an offset error in the measured optical spectrum.
Industrial applications requiring the measurement of an optical spectrum typically have some means to perform a reference measurement. In some cases this is performed manually: a reference sample is introduced into the field-of-view of the sampling head and measured. Subsequent sample measurements are divided by the reference in order to compute the desired optical spectrum. Another approach is available when fiber optics are used to deliver collected illumination: a fiber optic multiplexer provides the means to alternately view the illumination collected from the reference and sample(s). Such a multiplexer has multiple ports used for collected illumination inputs (e.g. reference plus one or more sample inputs) plus the means to direct the illumination from one of these inputs to a single output port. The illumination received at the output port is then delivered to the spectroscopic instrument.
Although present devices are functional, they are not sufficiently accurate or otherwise satisfactory. Accordingly, a system and method are needed to address the shortfalls of present technology and to provide other new and innovative features.